The invention relates to a driver circuit for controlling a high power load such as a actuator driver for CD or CD-ROM and a power amplifier for audio systems, and more particularly, to a current detector for use in such load driver circuits.
Metal oxide semiconductor field effect transistors (MOSFETs) are widely used as output stage elements and control elements of driver circuits controlling high power loads such as a actuator driver for CD or CD-ROM and a power amplifier for audio systems. These MOSFETs are mostly controlled by controlling their gate voltages.
FIGS. 1 and 2 show typical load driver circuits having a single bridge structure (FIG. 1) and a three-phase bridge structure (FIG. 2).
Load driving MOSFETs Q1-Q4 of the load driver circuit shown in FIG. 1 are controlled for conduction thereof by respective gate control signals 52 sent from a control circuit 51. Normally, the load driving MOSFETs Q1 and Q4 are simultaneously turned ON in pairs to provide a load current Io in one direction or another in the bridge. So are the load driving MOSFETS Q2 and Q3. The load current Io passing through a resistor for determining the current through a load L (referred to as current detection resistor Rs) generates a voltage drop across the resistor Rs, which is equal to the product Io*Rs. This voltage drop is amplified A times, say, by an amplifier 53, which generates a detection voltage Vo.
The detection voltage Vo is fed back to the control circuit 51 along with a reference voltage Vi. The control circuits supplies gate control signals 52 to the load driving MOSFETs Q1-Q4. The gate control signals 52 can be voltage control signals or pulse control signals such as pulse width modulation (PWM) control signals. Through the feedback loop the load current Io through the load L is adjusted to a constant level associated with the reference voltage Vi until the detection voltage Vo matches the reference voltage Vi.
The load driver circuit of FIG. 2 includes three loads LU, LV, and LW operable in three different phases and six load driving MOSFETs QU, QV, QW, QX, QY, and QZ forming an associated three-phase bridge for switching the three loads. Coupled to the three phase bridge is a control circuit 61 generating suitable three-phase gate control signals 62 for the three-phase bridge. An amplifier 63 is similar to the amplifier 53 of FIG. 1, and has a given amplification factor of A, for example. Since the load driver circuit of FIG. 2 operates in the same manner as the one shown in FIG. 1, further details of the circuit will be omitted.
It is noted that in such conventional load driver circuits as described above, the current detection resistor Rs is connected to a power source or the ground in series with the load in order to detect the load current through the load L or one of LU, LV, and LW. The load current Io is normally in the range from the order of 100 mA to the order of 1 A, and the resistance of the current detection resistor Rs for generating a measurable detection voltage is in the range from the order of 0.1 ohms to the order of 1 ohm.
Therefore, the power consumed by the current detection resistor Rs itself is disadvantageously large. That is, it is not negligible as compared with the entire power consumption. In addition, since such nominally large load current Io passes the detection resistor Rs, the resistor must be fairly large in dimension, requiring an extra large space for connection thereof with the driver circuit on a semiconductor chip.
Another drawback pertinent to such current detection resistor is that since the current detection resistor Rs is inserted in series with the load, the output dynamic range of the load is limited by the current detection resistor Rs.
It is therefore an object of the invention to provide a load driver circuit including at least one FET such as MOSFET for driving a load, said MOSFET controlled by a control circuit providing gate control voltage to the load driving FET by means of a feedback loop connecting the FET and the control circuit, without utilizing any resistor (referred to as detection resistor) connected in series with the load to detect the load current. Because the invention utilizes a MOSFET instead of a resistor in detecting the load current, the control circuit:
(a) has a compact form, requiring only a minimum space for connection with external components;
(b) cuts down significant power loss that would be otherwise entailed by a current detection resistor;
(c) is capable of detecting the load current at high precision, thereby allowing precise control of the load current; and
(d) provides an improved output dynamic range.
In accordance with one embodiment of the invention, a load driver circuit comprises:
at least one load driving MOSFET;
means for detecting the level of a load current including:
a detection transistor;
a detection resistor connected in series with said detection transistor for detecting a voltage drop across said detection resistor; and
a difference amplifier for controlling said detection transistor, said difference amplifier fed at one input end thereof with a voltage drop across the drain and the source of said load driving MOSFET in conduction, and at the other input end thereof with said voltage drop across said detection resistor; and
control means for receiving a signal associated with said voltage drop across the drain and the source of said load driving MOSFET along with a reference value indicative of a target level for said signal, and generating gate control signal to said load driving MOSFET to thereby equilibrate said level of said signal with said reference value.
In actuality, the control means described above may receive a reference voltage to compare with the voltage drop across the drain and the source of the load driving MOSFET, or a reference current with which the load current Io as calculated from the detected voltage drop across the drain and the source of the load driving MOSFET is compared.
The inventive load driver circuit makes it unnecessary to use a current detection resistor in the driver circuit by detecting the voltage drop across the drain and the source of the load driving MOSFET. This is based on the fact that an excellent linear relationship exists between the current passing through the MOSFET and the voltage drop across the drain and the source of the load driving MOSFET over a certain range of voltage, so that the effective resistance is constant/invariable, which enables accurate detection of the voltage drop across the load.
It should be appreciated that no current detection resistor is required in the invention, so that power consumption due to such current detection resistor as is used in conventional load driver circuit is zero. In addition, no mounting space for a terminal for connection with such resistor is required on the semiconductor chip.
The elimination of a current detection resistor to be connected in series with the load and a load driving MOSFET as in conventional driver circuits widens the working range of voltage of the load, and hence the dynamic range of the driver circuit.
The means for detecting the level of a load current includes a detection transistor, a detection resistor, and a difference amplifier such that the current through the detection resistor (referred to as detection current) is far smaller than the load current, yet it is precisely proportional to the load current. Consequently, power consumption by the detection resistor can be made very small. Further, since the detection resistor can be fabricated on the same semiconductor chip as the load driving MOSFET, they can have the same temperature dependent characteristics.
In cases where the control circuit includes a multiplicity of load driving MOSFETs forming a multi-phase bridge configuration, they can be selectively turned on at different times or phases by a switching means to provide their voltage drops across the drain and the source to the difference amplifier in the respective phases. Thus, the bridge configuration may practically serve as single detection bridge.
In accordance with another embodiment of the invention, a load driving circuit comprises:
at least one load driving MOSFET;
means for detecting the level of a load current including:
a detection transistor;
a detection MOSFET serving as a detection resistor connected in series with said detection transistor for detecting a voltage drop across said detection MOSFET; and
a difference amplifier for controlling said detection transistor, said difference amplifier fed at one input end thereof with a voltage drop across the drain and the source of said load driving MOSFET in conduction, and at the other input end thereof with said voltage drop across said detection MOSFET; and
control means for receiving a signal associated with said voltage drop across the drain and the source of said load driving MOSFET along with a reference value indicative of a target level for said signal, and generating a gate control signal to said load driving MOSFET to thereby equilibrate said level of said signal with said reference value.
As in the preceding example, the control means described above may receive a reference voltage to compare with the voltage drop across the drain and the source of the load driving MOSFET, or a reference current with which the load current Io as calculated from the detected voltage drop across the drain and the source of the load driving MOSFET is compared.
In this arrangement, the means for detecting the level of a load current includes a detection transistor, a detection MOSFET, and a difference amplifier such that the current through the detection MOSFET (referred to as detection current) is far smaller than the load current, yet it is precisely proportional to the load current. Consequently, power consumption by the detection MOSFET can be made very small. Further, since the detection MOSFET is fabricated on the same semiconductor chip as the load driving MOSFET, they will have the same temperature dependent characteristics or qualitative fluctuations induced during the manufacturing process. Therefore, their resistances will not be influenced by such fluctuations, so that the detection voltage will be precisely proportional to the load current.
In this instance also, if the control circuit includes a multiplicity of load driving MOSFETs forming a bride configuration, they can be selectively turned on at different times or phases by a switching means to provide their voltage drops across the drain and the source to the difference amplifier in the respective phases. Thus, the bridge configuration may practically serve as single detection means.
In accordance with a further embodiment of the invention, a load driving circuit comprises:
at least one load driving MOSFET;
a detection MOSFET serving as a detector resistor;
means for detecting the level of a load current, wherein said detecting means is operable to:
sense a voltage drop across the drain and the source of said load driving MOSFET;
sense a voltage drop across the drain and the source of said detection MOSFET; and
control a current passing through the detection MOSFET so that said voltage drop across the drain and the source of said detection MOSFET equals said voltage drop across the drain and the source of said load driving MOSFET; and
control means for receiving a signal associated with said voltage drop across the drain and the source of said load driving MOSFET along with a reference value indicative of a target level for said signal, and generating a gate control signal to said load driving MOSFET to thereby equilibrate said level of said signal with said reference value.
As in the preceding examples, the control means described above may receive a reference voltage to compare with the voltage drop across the drain and the source of the load driving MOSFET, or a reference current with which the load current Io as calculated from the detected voltage is compared.